The present invention relates to reflective liquid crystal devices and more particularly to reflective liquid crystal display device comprising a bistable twisted nematic (BTN) liquid crystal.
Reflective liquid crystal devices typically comprise a linear polariser and a mirror, with one or more retarders and a switchable liquid crystal element sandwiched between the polariser and the mirror. In a first state of the liquid crystal element, linearly polarised light passing through the polariser is reflected from the mirror and arrives back at the polariser with the same linear polarisation. Hence the reflected light is transmitted by the polariser and the device appears in the bright state. In a second state of the liquid crystal element, linearly polarised light is converted to have a circular polarisation such that upon reflection at the mirror the xe2x80x9chandednessxe2x80x9d of the circular polarisation is changed (i.e. from right to left or from left to right). Light arriving back at the polariser is arranged to have a polarisation angle 90xc2x0 shifted from the axis of the polariser (either the transmission or the absorption axis of the polariser may be considered and both give identical results). Hence, the reflected light to not transmitted by the polariser and the device appears in the dark state. Reflective liquid crystal devices are attractive particularly for low power consumption applications.
GB9622733.5 describes a reflective liquld crystal display device comprising a polariser and a mirror between which are disposed several retarders. One of the retarders is a liquid crystal element whose optic axis is switchable so as to switch the device between a reflective state and a non-reflective state.
EP0018180 describes a liquld crystal cell comprising a cholesteric liquid crystal. The liquid crystal is bistable and can exist in either one of its two metastable states until specific stops are taken to cause a transition to the other metastable state. Energy is expended only when a transition occurs.
The article xe2x80x9cBistable Twisted Nematic Mode with One Polariser for Reflective Liquid Crystal Displaysxe2x80x9d, Kim, Yu and Lee, IDRC Asia Display 98, Seoul, Korea, p 763, describes a device having in sequence an input linear polariser, a quarter wave retarder, a BTN element, and a mirror.
The article xe2x80x9cReflective Bistable Twisted Nematic Liquid Crystal Displayxe2x80x9d, Xie and Kwok, Jpn J. Appl. Phys. Vol37, part 1, No. 5a (1998), p 2572, describes a device having an input polariser, a BTN mode and a reflector. The proposed device does not include any additional retarders and states that the contrast ratio measured is about 6:1.
WO98/48320 describes a reflective liquid crystal display device comprising a polariser and a mirror between which are disposed a half wave retarder, a quarter wave retarder, and a liquid crystal element.
Abstract 39.3 (published in the SID 1999 International Symposium, Seminar and Exhibition-Advance program, in advance of a conference to be held in San Jose, Calif. USA, May 16-21, 1999) titled xe2x80x9cReflective Single-Polariser Bistable Nematic LCD with Optimum Twistxe2x80x9d, Y. J. Kim and J. S. Patel, describes a single-polariser reflective LCD having a bistable twisted nematic mode. The twist angles of the two bistable states are 63.6xc2x0 and 423.6xc2x0.
The article xe2x80x9cFast bistable nematic display using monostable surface switchingxe2x80x9d, I. Dozov, M. Nobili and G. Durand, Appl. Phys. Lett. 70 (9), 3 Mar. 1997, describes a device having a two bistable states, namely a metastable uniform (untwisted) state and a half-turn (180xc2x0) twist state.
According to a first aspect of the present invention there is provided a reflective liquid crystal device comprising in sequence a linear polariser, a retarder arrangement comprising two retarders, and a reflector, wherein, in at least one state of the device, a first of said retarders acts to rotate linearly polarised light of wavelength xcex, and a second of the retarders acts to convert linealy polarised light of wavelength yxcex (where 0.7 less than y less than 1.3) to substantially circular polarised light, and wherein at least one of the said first and second retarders comprises a Bistable Twisted Nematic (BTN) liquid crystal.
The wavelength xcex is an operating wavelength of the reflective liquid crystal device and lies in the visible spectrum and is preferably in the range 400-700 nm, more preferably 420-600 nm, and more preferably still in the region 440-550 nm. Most preferably xcex is approximately 550 nm.
Preferably, the retarder comprises a BTN liquid crystal providing a retardation of nxcex/4.
According to a second aspect of the present invention there is provided a reflective liquid crystal device comprising in sequence a linear polariser, a retarder arrangement comprising two retarders, and a reflector, wherein a first of said retarders provides a retardation of substantially mxcex/2 and a second of the retarders provides a retardation of substantially nxcex/4 where m is an integer and n is an odd integer, and wherein at least one of the said first and second retarders comprises a Bistable Twisted Nematic (BTN) liquid crystal and is switchable between a first state in which the retarder provides a retardation of substantially mxcex/2 or nxcex/4 and a second state in which the retardation is substantially zero.
According to a third aspect of the present invention there is provided a reflective liquld crystal device comprising in sequence a linear polariser, a retarder arrangement comprising at least three retarders, and a reflector, wherein at least one of said retarders comprises a Bistable Twisted Nematic (BTN) liquid crystal and is switchable between first and second retardation states.
The wavelength xcex is an operating wavelength of the reflective liquid crystal device and lies in the visible spectrum and is preferably in the range 400-700 nm, more preferably 420-600 nm, and more preferably still in the region 440-550 nm. Most preferably xcex is approximately 550 nm.
Preferably, the retarder comprising a BTN liquid crystal providing a retardation in said first state of substantially mxcex/2 or nxcex/4 where m is an integer and n is an odd integer, and a retardation in said second state of substantially zero. The others of said retarders may provide a retardation of substantially mxcex/2 or nxcex/4.
xe2x80x9cOptic axisxe2x80x9d means slow optic axis.